Inflators in vehicles provide the filling gas required for filling an airbag or else the propellant required to drive a belt retractor. The gas can be generated in the inflator by combustion of solid fuel into a gaseous end product, by releasing compressed gas stored in the inflator, by combustion of compressed gas or by a combination of different of these processes.
For reasons of operational safety it is advantageous to use inert gases in compressed inflators. In this context, above all argon, helium and nitrogen as well as mixtures of said gases are employed.
The inert gas is stored at a pressure of e.g. 500 bars in a compressed gas container which is closed against the ambience by a bursting membrane. The latter is opened upon activation of the inflator so that the gas can be discharged from the inflator.
Upon activation of the inflator, a small pyrotechnical propellant contained in an igniter which releases a particular amount of gas and causes a particular increase in temperature is usually ignited. The increase in temperature can be exploited to heat the compressed gas such that the internal pressure in the compressed gas container exceeds the bursting pressure of the bursting membrane. However, this requires a relatively large amount of pyrotechnical fuel, means a relatively high time delay between igniting the propellant and opening the bursting membrane and, moreover, results in the fact that the discharging gas is very hot and further measures have to be taken to protect the airbag against the hot gases.
A different technology turned out to be advantageous in which the igniter is used to generate a shock wave entering the compressed gas container and running through the same up to the bursting membrane, wherein a local extremely short increase in pressure exceeds the bursting pressure of the bursting membrane only in the area of the shock wave and the wave front thereof and opens the bursting membrane. A shock wave typically runs through the medium at high velocity which may be above sonic velocity. The medium is strongly compressed in the area of the wave front of the shock wave in a spatially closely limited area so that in this area the pressure is strongly increased vis-a-vis the mean pressure of the medium. In this way early opening is effectuated after activation. With this technology, the mean temperature in the compressed gas container until opening is definitely below that of the first described method.
However, this technology at present strongly restricts the geometries usable for the inflator. Moreover, there are problems in using multi-stage inflators in which the amount of gas generated is to be controlled.